Extremely high radiation levels have been recorded inside a damaged reactor at the Fukushima Daiichi nuclear power station, almost six years after the plant suffered a triple meltdown.

The facility’s operator, Tokyo Electric Power (Tepco), said atmospheric readings as high as 530 sieverts an hour had been recorded inside the containment vessel of reactor No 2, one of three reactors that experienced a meltdown when the plant was crippled by a huge tsunami that struck the north-east coast of Japan in March 2011.

The extraordinary radiation readings highlight the scale of the task confronting thousands of workers, as pressure builds on Tepco to begin decommissioning the plant – a process that is expected to take about four decades.

Even if a 30-percent margin of error is taken into account, the recent reading, described by some experts as “unimaginable”, is far higher than the previous record of 73 sieverts an hour detected by sensors in 2012.

Tepco pointed out, however, that the camera had probed deeper inside the reactor than before and had focused on a single point. Radiation levels at other spots filmed by the camera are estimated to be much lower, it added.

A single dose of one sievert is enough to cause radiation sickness and nausea; 5 sieverts would kill half those exposed to it within a month, and a single dose of 10 sieverts would prove fatal within weeks.

Tepco also said image analysis had revealed a hole in metal grating beneath the same reactor’s pressure vessel. The one-metre-wide hole was probably created by nuclear fuel that melted and then penetrated the vessel after the tsunami knocked out Fukushima Daiichi’s back-up cooling system.

“It may have been caused by nuclear fuel that would have melted and made a hole in the vessel, but it is only a hypothesis at this stage,” Tepco’s spokesman Tatsuhiro Yamagishi told AFP.

“We believe the captured images offer very useful information, but we still need to investigate given that it is very difficult to assume the actual condition inside.”

The presence of dangerously high radiation will complicate efforts to safely dismantle the plant.

A remote-controlled robot that Tepco intends to send into the No 2 reactor’s containment vessel is designed to withstand exposure to a total of 1,000 sieverts, meaning it would survive for less than two hours before malfunctioning.

The firm said radiation was not leaking outside the reactor, adding that the robot would still prove useful since it would move from one spot to the other and encounter radiation of varying levels.

Tepco and its network of partner companies at Fukushima Daiichi have yet to identify the location and condition of melted fuel in the three most seriously damaged reactors. Removing it safely represents a challenge unprecedented in the history of nuclear power.

Quantities of melted fuel are believed to have accumulated at the bottom of the damaged reactors’ containment vessels, but dangerously high radiation has prevented engineers from accurately gauging the state of the fuel deposits.

Earlier this week, the utility released images of dark lumps found beneath reactor No 2 that it believes could be melted uranium fuel rods – the first such discovery since the disaster.

In December, the government said the estimated cost of decommissioning the plant and decontaminating the surrounding area, as well as paying compensation and storing radioactive waste, had risen to 21.5tn yen (£150bn), nearly double an estimate released in 2013.

This article was amended on 9 February to explain that the camera had probed deeper inside the No 2 reactor – and closer to the damaged nuclear fuel – than before, hence the high radiation estimate.